Synthesis of high-surface-area nanoporous BiVO4 electrodes

ABSTRACT

Photoelectrochemical materials and photoelectrodes comprising the materials are provided. The photoelectrochemical materials comprise a porous, high-surface-area BiVO 4  that is composed of particles smaller than the hole diffusion length of BiVO 4 .

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under 1305124 awarded bythe National Science Foundation. The government has certain rights inthe invention.

BACKGROUND

N-type bismuth vanadate (BiVO₄) has recently emerged as a promisingphotoanode for use in water-splitting photoelectrochemical cells becauseit absorbs a substantial portion of the visible spectrum and has afavorable conduction band (CB) edge position very near the thermodynamicH₂ evolution potential. However, the solar-to-hydrogen conversion (STH)efficiency achieved with BiVO₄ to date has been far below what isexpected because the material suffers from poor electron-hole separationyield (φ_(sep)). Previous efforts to improve the φ_(sep) of BiVO₄ mainlyfocused on doping studies, which were intended to improve its poorelectron transport properties.

A porous BiVO₄ material has been made by applying V₂O₅ dissolved inaqueous NH₄OH solution onto a BiOI substrate, followed by conversion toBiVO₄. However, because the hydrophilic V₂O₅-containing solution couldnot wet the hydrophobic surface of BiOI, the resulting BiVO₄ electrodeswere composed of large submicron size particles, limiting the surfacearea and photoelectrochemical performance of the material. (See McDonaldet al., Energy Environ. Sci. 5, 8553-8557 (2012).)

SUMMARY

Photoelectrochemical materials and photoelectrodes comprising thematerials are provided. Also provided are photoelectrochemical cells(PECs) and methods for using the photoelectrochemical cells.

One embodiment of a photoelectrochemical material comprises a porousnetwork of BiVO₄ particles, wherein the BiVO₄ particles have a meanparticle size of less than about 150 nm and the porous network has asurface area of at least about 10 m²/g, as measured by theBrunauer-Emmett-Teller method.

Another embodiment of a photoelectrochemical material comprises a porousnetwork of BiVO₄ particles and a coating of oxygen evolution catalyst onthe surfaces of the BiVO₄ particles. The coating of oxygen evolutioncatalyst comprises an inner layer of a first oxygen evolution catalystin direct contact with the surfaces of the BiVO₄ particles and an outerlayer of a second oxygen evolution catalyst disposed on the inner layer.The first oxygen evolution catalyst is characterized in that it createsfewer interfacial states that can serve as recombination centers at theBiVO₄/oxygen evolution catalyst interface than would the second oxygenevolution catalyst and the second oxygen evolution catalyst ischaracterized in that it has higher catalytic activity for oxygenevolution than does the first oxygen evolution catalyst.

One embodiment of a photoelectrochemical cell comprises: a workingphotoelectrode comprising a porous network of BiVO₄ particles, whereinthe BiVO₄ particles have a mean particle size of less than about 150 nmand the porous network has a surface area of at least about 10 m²/g, asmeasured by the Brunauer-Emmett-Teller method; a counter electrode inelectrical communication with the working photoelectrode; and anelectrolyte solution in which the working photoelectrode and the counterelectrode are immersed.

One embodiment of a method for carrying out electrochemical reactionsusing a photoelectrochemical cell, of the type described above,comprises: exposing the photoelectrode to radiation having energygreater than the bandgap of the BiVO₄, such that: the BiVO₄ particlesabsorb the radiation to produce electron-hole pairs; the holes aretransported to the electrolyte-photoelectrode interface where theyundergo oxidation reactions with the electrolyte; and the electrons aretransported to the counter electrode where they undergo reductionreactions with the electrolyte.

One embodiment of a method of making a photoelectrochemical materialcomprises: applying an organic solution comprising vanadium-containingprecursor molecules to the surface of a porous network of plate-likeBiOX crystals, where X represents a halogen atom or a mixture of halogenatoms; and heating the applied organic solution and the porous networkof plate-like BiOX crystals to convert the porous network of plate-likeBiOX crystals into a porous network of BiVO₄ particles; wherein theBiVO₄ particles have a mean particle size of less than about 150 nm andthe porous network of BiVO₄ particles has a surface area of at leastabout 10 m²/g, as measured by the Brunauer-Emmett-Teller method.

Another embodiment of a method of making a photoelectrochemical materialcomprises: exposing BiVO₄ to a reducing atmosphere comprising N₂ at anelevated temperature for a time sufficient to create oxygen vacanciesand substitutional defects in which oxygen atoms are replaced withnitrogen atoms in the BiVO₄.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIG. 1. Schematic representation of the synthesis procedure used in theExample.

FIG. 2. Morphologies of nanoporous BiVO₄ electrodes. (A) SEM image ofBiOI. (B and C) Top view and side view SEM images of a BiVO₄ electrodeprepared using NH₄OH/V₂O₅. (D to F) Top view and side view SEM images ofnanoporous BiVO₄ prepared using DMSO/VO(acac)₂.

FIG. 3. Effect of OECs on photocurrents for water oxidation and sulfiteoxidation. (A) J-V curves of BiVO₄ (gray solid line), BiVO₄/FeOOH (blackdashed), BiVO₄/NiOOH (black dot dashed), BiVO₄/FeOOH/NiOOH (blacksolid), and BiVO₄/NiOOH/FeOOH (black dot dot dashed) for water oxidationmeasured in a 0.5 M phosphate buffer (pH 7) under AM 1.5 G illumination.Dark current is shown as a gray dashed line. (B to E) J-V curves of (B)BiVO₄/FeOOH, (C) BiVO₄/NiOOH, (D) BiVO₄/FeOOH/NiOOH, and (E)BiVO₄/NiOOH/FeOOH comparing photocurrent for sulfite oxidation (graydashed) and water oxidation (gray solid) measured with and without thepresence of 1.0 M Na₂SO₃ as hole scavenger. Photocurrent for sulfiteoxidation by BiVO₄ is shown as the black dashed line for comparison. Themean values and SDs of photocurrent onset potentials and photocurrentdensities are summarized in Tables 2 and 3.

FIG. 4. Photoelectrolysis of water by BiVO₄/FeOOH/NiOOH photoanode. (A)STH obtained using a two-electrode system. (B) J-t curve measured at 0.6V versus counter electrode in a phosphate buffer (pH 7) under AM 1.5 Gillumination. (C) Detection of H₂ and O₂ at 0.6 V versus counterelectrode.

FIG. 5. Particle size distribution of BiVO₄ in the nanoporous BiVO₄electrode obtained from measuring diameters of 400 particles shown inSEM images.

FIG. 6. N₂ adsorption-desorption measurement and pore size analyses fora nanoporous BiVO₄ electrode; (A) BET isotherms and pore sizedistributions calculated by (B) BJH and (C) MP methods.

FIG. 7. Photoelectrochemical properties of a nanoporous BiVO₄ electrodefor sulfite oxidation. (A) J-V curve of the nanoporous BiVO₄ electrodemeasured in a 0.5 M phosphate buffer (pH 7) containing 1 M Na₂SO₃ as ahole scavenger under AM 1.5 G, 100 mW/cm² illumination (scan rate, 10mV/s). Dark current is shown as a dashed line. (Inset) φ_(sep)calculated from the J-V curve after dark current is subtracted. (B) IPCE(circles) and APCE (triangles) measured in the same solution at 0.6 Vversus RHE.

DETAILED DESCRIPTION

Photoelectrochemical materials and photoelectrodes comprising thematerials are provided. Also provided are photoelectrochemical cells(PECs) and methods for using the photoelectrochemical cells inwater-splitting applications. The photoelectrochemical materialscomprise a porous, high-surface-area BiVO₄ that is composed of particleshaving a mean particle size that is comparable to or smaller than thehole diffusion length of BiVO₄. As a result, the materials havesignificantly increased φ_(sep), relative to other BiVO₄-basedphotoelectrochemical materials, even in the absence of extrinsic doping.In some embodiments, the BiVO₄ is formed with oxygen vacancies andsubstitutional defects in which oxygen atoms are replaced with nitrogenatoms in order to improve the photoelectrochemical performance of thematerial.

The photoelectrodes are characterized by high STH efficiencies,including STH efficiencies of 1.5% or greater. In some embodiments, thephotoelectrodes provide an STH efficiency of at least 1.7%. Thisincludes embodiments in which the photoelectrodes provide an STHefficiency of at least 2% (e.g., from about 2% to about 3%). Methods formeasuring the STH efficiencies of the cells are described in theExample.

In addition, a dual-layer oxygen evolution catalyst (OEC) is provided.The catalyst is provided as a coating on the BiVO₄ material and improvesthe efficiency of water oxidation in a PEC. The two layers that make upthe catalyst are designed to optimize the interface between the porousBiVO₄ material and the OEC coating, as well as the interface between theOEC coating and the electrolyte of the PEC.

One embodiment of the photoelectrochemical materials comprises a porousnetwork of BiVO₄ particles, wherein the BiVO₄ particles have a meanparticle size of less than about 150 nm and the porous network has asurface area of at least about 10 m²/g. In some embodiments the porousnetwork has a surface area of at least about 25 m²/g. Unless otherwisespecified, for the purposes of this disclosure surface area refers tosurface area as measured by the Brunauer-Emmett-Teller (BET) method,which is described in the Example. In some embodiments of the materials,the BiVO₄ particles have a mean particle size of no greater than about100 nm. This includes embodiments in which the mean particle size of theBiVO₄ particles in the material is no greater than about 85 nm. By wayof illustration only, the BiVO₄ particles may have a mean particle sizein the range from 60 nm to 90 nm.

Particle size can be measured by obtaining representative scanningelectron microscope (SEM) images of the materials and measuring thediameter of a collection of the particles in the image. A sufficientnumber of particle diameters should be measured in order to arrive at anaccurate mean particle size for the material. This is illustrated in theExample, which describes the determination of the mean BiVO₄ particlesize for the material shown in the SEM image of FIG. 2F. The BiVO₄particles may be merged at their interfaces to form shapes havingbulbous portions, corresponding to the particles, that are connected bynarrower interparticle portions. For these multi-particle forms, thesize of the individual particles can be determined by measuring thediameter of the bulbous portions of the shape. Notably, in the presentmaterials, the interfaces at which the particles merge can be free ofgrain boundaries which tend to decrease carrier mobility.

The nanoporous materials may have an average interparticle pore diameterof less than about 1000 nm. For example, in some embodiments thematerials have an average interparticle pore diameter in the range fromabout 10 to about 500 nm. The average interparticle pore diameter can bedetermined by using the Barrett-Joyner-Halenda (BJH) formula, asillustrated in the Example. As used herein, the term interparticle porerefers to pores formed in the space between the particles. Interparticlepores are distinguishable from intraparticle pores, which exist withinthe particles.

The small mean particle size of the BiVO₄ particles that make up thematerials provides the materials with a high electron-hole separationyield, which renders them well-suited for use as electrode materials inphotoelectrochemical cells. A basic embodiment of a PEC comprises: aworking photoelectrode comprising a porous network of BiVO₄ particles,as described herein; a counter electrode in electrical communicationwith the working photoelectrode; and an electrolyte solution in whichthe working photoelectrode and the counter electrode are immersed.

Electron-hole separation yield is the yield of photogenerated holes thatreach the surface of the material before undergoing recombination.Because the sizes of the BiVO₄ particles are smaller than their holediffusion lengths, the materials have improved electron-hole separationyield relative to porous BiVO₄ materials composed of larger particles.Electron-hole separation yield can be measured for a photoanodecomprising the porous materials versus a reversible hydrogen electrode(RHE) using eq. (1):J _(PEC) =J _(abs)×φ_(sep)×φ_(ox)  Eq. (1)

where J_(PEC) is the measured photocurrent density for sulfiteoxidation, J_(abs) is i the photon absorption rate expressed as currentdensity and φ_(ox) is the yield of surface reaching holes that areinjected into solution. (A detailed description of how φ_(sep) can bedetermined using eq. (1) is provided in the Example.) Some embodimentsof photoanodes comprising the present photoelectrochemical materialsprovide a φ_(sep) of at least 0.7 at 1.23 V vs. RHE. This includesembodiments that provide a φ_(sep) of at least 0.8 and at least 0.9 at1.23 V vs. RHE. When measured at 0.6 V vs. RHE, these materials canprovide a φ_(sep) of at least 0.4. This includes embodiments thatprovide a φ_(sep) of at least 0.6 and at least 0.7 at 0.6 V vs. RHE.

The PECs can be used to carry out a variety of electrochemical reactionsby exposing the photoelectrode to radiation having energy greater thanthe bandgap of the BiVO₄, such that the BiVO₄ particles absorb theradiation to produce photogenerated electron-hole pairs. Thephotogenerated holes are transported to the electrolyte-photoelectrodeinterface where they undergo oxidation reactions with an electrolyte andthe photogenerated electrons are transported to the counter electrodewhere they undergo reduction reactions with the electrolyte. For examplein a water-splitting reaction, photogenerated holes that reach theelectrolyte-photoelectrode interface oxidize water in the aqueouselectrolyte solution to form O₂, while photogenerated electrons at thecounter electrode reduce water in the aqueous electrolyte solution toform H₂.

If the PEC is intended for use in water-splitting applications, it maybe advantageous to coat the photoelectrode with an OEC. OECs areelectrocatalysts for the oxygen evolution reaction at the photoanode andare used to improve the oxidation kinetics. These coatings are typicallyquite thin and may be continuous or discontinuous. Examples of suitableOECs include oxyhydroxides, hydroxides and oxides of nickel and iron.These compounds can be represented by the formulas NiO_(x)(OH)_(y)(0≦x≦1.5, 0≦y≦3, where at least one of x and y has a value >0) andFeO_(x)(OH)_(y) (0≦x≦1.5, 0≦y≦3, where at least one of x and y has avalue >0), respectively, and are also referenced as NiOOH and FeOOH. Oneaspect of the present technology provides a multi-layered OEC coatingthat includes an inner layer of a first OEC in direct contact with thesurface of the BiVO₄ particles and an outer layer of a second OEC thatis in direct contact with the aqueous electrolyte of a PEC. The firstOEC is characterized in that it creates fewer interfacial states withinthe bandgap of the BiVO₄ than would the second OEC. Reducing interfacialstates within the bandgap is advantageous because it reduces therecombination pathways for photogenerated holes. The second OEC ischaracterized in that it has higher catalytic activity for oxygenevolution than does the first OEC. The structure of the materials thatmake up the first and second OECs should be sufficiently similar thatthe gains from the reduction in interfacial states at the BiVO₄/OECinterface are not lost due to the formation of interfacial states at the1^(st) OEC/2^(nd) OEC interface. The net effect of the dual-layer OEC isthat the photoanode coating with the dual-layer OEC has a higher wateroxidation efficiency than the same photoanode coated with a single-layerOEC made from either the first or the second OEC alone. In someembodiments of the dual-layer OEC, the inner OEC layer comprises FeOOHand the outer OEC layer comprises NiOOH.

The fabrication of the photoelectrochemical materials is illustratedschematically in FIG. 1. As shown in that figure, the materials can bemade by applying a non-aqueous organic solution 104 comprisingvanadium-containing precursor molecules to the surface of a porousnetwork of plate-like BiOX crystals 102 that are disposed on a substrate100 (step (a)), where X represents a halogen atom, such as I, Br or Cl,or a mixture of two or more halogen atoms. The applied organic solutionand the porous network of plate-like BiOX crystals are then heated to anelevated temperature for a time sufficient to convert the porous networkof plate-like BiOX crystals into a porous network of BiVO₄ particleswith excess V₂O₅ 106 (step (b)). The excess V₂O₅ can be dissolved using,for example, NaOH to provide a pure, porous network of BiVO₄ particles108 (step (c)). The advantage of using a BiOX, such as BiOI, is that itstwo dimensional (2D) crystal structure enables electrodeposition ofextremely thin plates (˜20 nm) with sufficient voids between them (FIG.2A). These voids inhibit grain growth of BiVO₄ during the conversionprocess, resulting in nanoporous BiVO₄ electrodes. By way ofillustration only, elevated temperatures include temperatures in therange from about 300 to about 700° C. and sufficient times include timesin the range from about 1 to about 6 hours.

One or more organic solvents may be used to make the organic solution.It is desirable to use relatively hydrophobic solvents to achieve goodwetting of the BiOX crystal network, which provides a uniformdistribution of the vanadium-containing precursor molecules on thesurface of the structure. Dimethyl sulfoxide (DMSO) is an example of asuitable organic solvent. Other suitable organic solvents includepropylene carbonate, formamide, ethyleneglycol, dimethylformamide,acetonitrile, acetylacetone, diethyl allylmalonate, cinnamyl acetate,benzene, ethanol, acetic acid. The precursor molecules include vanadiumoxides, other inorganic vanadium oxides, and metallorganic compounds inwhich the metal is vanadium. Vanadyl acetylacetonate (VO(acac)₂) is oneexample of a suitable vanadium-containing precursor. Other suitablevanadium-containing precursor molecules include NH₄VO₃, VO, V₂O₃, VO₂,V₂O₅, VCl₂, VCl₃, V(C₂H₃O₂)₃, VOSO₄.

Once the BiVO₄ material has been fabricated in this manner, it may besubjected to a heat treatment in a reducing or an inert atmosphere. Thistreatment step, which can be conducted at elevated temperatures under aflow of an a reducing or an inert gas, creates oxygen vacancies in theBiVO₄. This is advantageous because the vacancies effectively act likedopants to further improve electron-hole separation in the BiVO₄.Examples of gases that can be used as reducing or inert gases includeN₂, H₂, CO, NH₃, Ar, and mixtures thereof. The use of N₂ is advantageousbecause, in addition to forming oxygen vacancies, nitrogen treatmentcreates substitutional defects in the BiVO₄ in which a portion of theoxygen atoms are replaced by nitrogen atoms. This is advantageousbecause the nitrogen reduces the bandgap of the BiVO₄ so that thematerial can absorb a broader spectrum of solar radiation, therebyimparting PECs incorporating the electrodes with greater STHefficiencies. For example, the bandgap may be reduced to a value of lessthan 2.5 eV (e.g., a value in the range from 2.0 to 2.4 eV). By way ofillustration, some embodiments of PECs incorporating the treatedphotoelectrodes provide an STH efficiency of at least 2% (e.g., fromabout 1.8 to about 3%). This includes embodiments of the PECs thatprovide as STH efficiency of at least 2.0%. Although the nitrogentreatment is described here in conjunction with the formation ofphotoelectrochemical materials comprising a porous network of BiVO₄particles, wherein the BiVO₄ particles have a mean particle size of lessthan 150 nm, it can be used to improve the photoelectrochemicalperformance of other BiVO₄-based materials as well.

Once the BiVO₄ electrode has been fabricated, a coating comprising oneor more OECs can be deposited onto its surface via, for example,photodeposition or electrodeposition from solution, as described in theexample that follows.

EXAMPLE

This example illustrates the fabrication of a high-surface-areananoporous BiVO₄ electrode composed of particles having a mean particlesize that is smaller than the hole diffusion length of the BiVO₄. Theresults presented here demonstrate that the electrode can effectivelyincrease φ_(sep) without additional doping. Furthermore, these studiesinvestigated the effect of an OEC layer on the interfacial recombinationat the BiVO₄/OEC junction and water oxidation kinetics using twodifferent OECs, FeOOH and NiOOH. The results illustrate the advantage ofdual layers of OECs that optimize both the BiVO₄/OEC and theOEC/electrolyte junctions simultaneously, to enable efficientutilization of surface-reaching holes for solar water oxidation.

Materials and Methods

Electrodeposition of BiOI Electrodes

A 0.04 M Bi(NO₃)₃ solution was prepared by dissolving Bi(NO₃)₃.5H₂O in50 mL of a 0.4 M KI solution after its pH was adjusted to 1.7 by addingHNO₃, according to the procedure described in McDonald et al., EnergyEnviron. Sci. 5, 8553-8557 (2012). This solution was mixed with 20 mL ofabsolute ethanol (100%) containing 0.23 M p-benzoquinone, and wasvigorously stirred for a few minutes. A typical three-electrode cell wasused for electrodeposition. A fluorine-doped tin oxide (FTO) workingelectrode (WE), a Ag/AgCl (4 M KCl) reference electrode (RE), and aplatinum counter electrode (CE) were used. The platinum CE was preparedby depositing 100 nm platinum on top of a 30 nm titanium adhesion layeron a cleaned glass slide by e-beam evaporation. A VMP2 multichannelpotentiostat (Princeton Applied Research) was used for electrodepositionand subsequent electrochemical studies. Cathodic deposition wasperformed potentiostatically at −0.1 V vs. Ag/AgCl at RT with varyingdeposition times (1-30 min). The optimum deposition time was found to be3-5 min, which was equivalent to passing a total charge of 0.13 C/cm².Under cathodic bias, p-benzoquinone is reduced to hydroquinone,elevating the local pH on the WE (shown below), giving rise to theprecipitation of crystalline BiOI on the WE.

Synthesis of BiVO₄ Electrodes

0.15-0.2 mL of a dimethyl sulfoxide (DMSO) solution containing 0.2 Mvanadyl acetylacetonate (VO(acac)₂) was placed on the BiOI electrode (1cm×1.3 cm) (FIG. 1) and was heated in a muffle furnace at varyingtemperatures (350-650° C.) and for varying durations (2-5 h) in air toconvert BiOI to BiVO₄. BiVO₄ electrodes showing the highest photocurrentwere obtained when BiOI electrodes were annealed at 450° C. (rampingrate=2° C./min) for 2 h. Excess V₂O₅ present in the BiVO₄ electrodes wasremoved by soaking them in 1 M NaOH solution for 30 min with gentlestirring. The resulting pure BiVO₄ electrodes were rinsed with deionized(DI) water and dried at room temperature (RT).

A non-aqueous dimethyl sulfoxide (DMSO) solution containing vanadylacetylacetonate, VO(acac)₂, as the vanadium source was used. DMSO isrelatively hydrophobic. And, in particular, is less hydrophilic thatpreviously used aqueous solutions of V₂O₅ in NH₄OH. This ensured evensupply of the vanadium source throughout the BiOI electrode, whichinduced multiple BiVO₄ nucleation processes within a single 2D BiOIsheet, taking apart the sheet and introducing more porosity along thez-axis. The SEM images in FIG. 2 provide a comparison of themorphologies of electrodes made using a solution of V₂O₅ in NH₄OH (FIGS.2B and 2C) and using the present solution of VO(acac)₂ in DMSO (FIGS. 2Dand 2F).

Photodeposition of FeOOH/NiOOH OECs on BiVO₄ Electrodes

(1) Photodeposition of FeOOH on BiVO₄.

Photodeposition of FeOOH on the BiVO₄ electrode was carried out in a 0.1M FeSO₄ solution while gently stirring. Prior to the photodeposition ofFeOOH, the solution was purged with nitrogen gas for 1 h. An undividedthree-electrode cell was used, which was composed of a BiVO₄ WE, a PtCE, and a Ag/AgCl (4 M KCl) RE. A 300 W Xe arc lamp with an AM 1.5 Gfilter, neutral density filters, and a water filter (IR filter) was usedas the light source. The light was illuminated through the FTO contact(back-side illumination) and the light intensity at the FTO surface was1 mW/cm². During illumination, the holes generated in the valence bandof BiVO₄ were used to oxidize Fe²⁺ ions to Fe³⁺ ions, which precipitateas FeOOH on the surface of the BiVO₄ electrode (Fe²⁺(aq)+h⁺+3OH⁻+→FeOOH(s)+H₂O). (K. J. McDonald, K.-S. Choi, A newelectrochemical synthesis route for a BiOI electrode and its conversionto a highly efficient porous BiVO₄ photoanode for solar water oxidationEnergy Environ. Sci 5, 8553-8557 (2012) and J. A. Seabold, K.-S. Choi,Efficient and stable photo-oxidation of water by a bismuth vanadatephotoanode coupled with an iron oxyhydroxide oxygen evolution catalyst.J. Am. Chem. Soc. 134, 2186-2192 (2012)) To facilitate photodeposition,an external bias of ca. 0.25 V vs. Ag/AgCl (4 M KCl), which was the opencircuit potential of the BiVO₄ electrode in the solution in dark, wasapplied. Various deposition times (i.e., 5, 10, 20, 40, and 60 min) weretested and the BiVO₄/FeOOH electrode with FeOOH deposited for ˜20 min(equivalent to passing a total charge of ˜45 mC/cm²) showed the highestphotocurrent. After photodeposition, an electrodeposition of FeOOH wascarried out in the same solution by applying +1.2 V vs. Ag/AgCl (4 MKCl) for 1 min. This was to deposit FeOOH on any bare BiVO₄ or FTOsurfaces exposed to the electrolyte.

(2) Photodeposition of NiOOH on BiVO₄.

The same procedure used for the photodeposition of FeOOH was used exceptfor the plating solution, which was a 0.1 M NiSO₄ solution with pHadjusted to 6.0-7.2 by carefully adding NaOH. During illumination, likethe photodeposition of FeOOH, photogenerated holes in BiVO₄ oxidizedNi²⁺ to Ni³⁺ ions, and Ni³⁺ ions precipitate as NiOOH (Ni²⁺(aq)+h⁺+3OH⁻→NiOOH(s)+H₂O). (L. Zhang, Y. Zhong, Z. He, J. Wang, J. Xu,J. Cai, N. Zhang, H. Zhou, H. Fan, H. Shao, J. Zhang, C.-Cao,Surfactant-assisted photochemical deposition of three-dimensionalnanoporous nickel oxyhydroxide films and their energy storage andconversion properties. J. Mater. Chem. A 1, 4277-4285 (2013).) Tofacilitate photodeposition, an external bias of ca. 0.11 V vs. Ag/AgCl(4 M KCl), which was the open circuit potential of the BiVO₄ electrodein the solution in dark, was applied. The highest photocurrent densityof BiVO₄/NiOOH was obtained when NiOOH was deposited by passing a totalcharge of ˜22 mC/cm². After photodeposition, electrodeposition of NiOOHwas carried out in the same solution by applying +1.2 V vs. Ag/AgCl (4 MKCl) for 1 min. (G. W. D. Briggs, M. Fleischmann, Anodic deposition ofNiOOH from nickel acetate solutions at constant potential. Trans.Faraday Soc. 62, 3217-3228 (1966).)

(3) Photodeposition of FeOOH/NiOOH.

In order to prepare BiVO4/FeOOH/NiOOH photoanode, a FeOOH layer wasfirst photodeposited on BiVO₄ followed by the photodeposition of NiOOHusing the optimum conditions described using the conditions described in(1) and (2). A total charge of ˜45 mC/cm² and ˜22 mC/cm² was passed forthe deposition of FeOOH and NiOOH, respectively. After photodepositionof NiOOH, additional electrodeposition of NiOOH was performed byapplying +1.2 V vs. Ag/AgCl (4 M KCl) for 1 min using the same solution.The atomic ratio of Fe/Ni in optimized BiVO₄/FeOOH/NiOOH electrode wasestimated to be 2.9±0.7 by EDS analysis. (The standard deviation wasobtained by averaging five different electrodes.)

(4) Photodeposition of NiOOH/FeOOH.

For the purpose of comparison with BiVO₄/FeOOH/NiOOH, BiVO₄/NiOOH/FeOOHwith a reversed OEC junction was also prepared by first photodepositingNiOOH on BiVO₄ followed by the photodeposition of FeOOH using theconditions described in (1) and (2). A total charge of ˜22 mC/cm² and˜45 mC/cm² was passed for the deposition of NiOOH and FeOOH,respectively. After photodeposition of FeOOH, additionalelectrodeposition of FeOOH was performed by applying +1.2 V vs. Ag/AgCl(4 M KCl) for 1 min using the same solution. The atomic ratio of Fe/Nifor the reverse BiVO₄/NiOOH/FeOOH electrode prepared was comparable 5 tothat of the BiVO₄/FeOOH/NiOOH.

Nitrogen Treatment

The nitrogen treatment of the BiVO₄ electrode was performed by heatingthe electrode in a tube furnace while flowing N₂. The tube furnace waspurged by flowing N₂ at 25° C. for 1 h and then the temperature wasraised and kept at 350° C. for 1 h (ramping rate. 6° C./min).

Characterization

The purity and crystal structure of BiOI and BiVO₄ electrodes wereexamined by powder X-ray diffraction (Bruker D8 Advanced PXRD, λ,=1.5418 Å, 298 K, Ni-filtered Cu Kα-radiation). The crystal morphologiesof the electrodes were examined with Scanning Electron Microscopy (SEM)using a LEO 1530 or a Hitachi S3400-N microscope at an acceleratingvoltage of 5-15 kV. The Bi/V ratio of the BiVO₄ electrode was confirmedto be 1.02±0.05 by optical emission spectrometry-inductively coupledplasma spectrometry (OES-ICP) (Perkin Elmer Optima 2000) and a HitachiS3400-N microscope equipped with an energy dispersive X-ray spectrometer(EDS) (Thermo Fisher Scientific Inc.), indicating the absence of any Bior V containing amorphous impurities that cannot be detected by XRD.UV-vis absorption spectra were obtained on a Cary 5000 UV-visspectrophotometer, in which the sample electrode was placed in thecenter of an integrating sphere to measure all light reflected andtransmitted to accurately assess the absorbance. FTO glass was used asthe reference for these absorption measurements. The surface area andpore structure of the present bare-BiVO₄ electrode were examined bymeasuring nitrogen adsorption-desorption isotherms at liquid nitrogentemperature (77 K) using a surface area analyzer (Micromeritics GeminiVII 2390). The sample was degassed at 200° C. for 12 h under vacuumprior to the adsorption measurements. The result shows that thenanoporous BiVO₄ displays the Brunauer-Deming-Deming-Teller (BDDT) typeI and type IV shape isotherms, along with H2 and/or H3-type hysteresisloop (at 0.4<pp_(o) ⁻¹<1) in IUPAC classification, suggesting thepresence of mesopores and macropores. (T. Allen, Particle SizeMeasurement (Chapman and Hall, London, 1997).) The sample also exhibitssteep N₂ adsorption at pp_(o) ⁻¹<0.05, indicating the presence of highconcentration of micropores. The mesopores and macropores representvoids present between BiVO₄ nanoparticles while the micropores representpores present within each BiVO₄ particle. The specific surface area ofnanoporous BiVO₄ electrode was estimated to be 31.8±2.3 m²/g based on afitting analysis using the Brunauer-Emmett-Teller (BET) equation (Table1). The zeta potential of BiVO₄, BiVO₄/FeOOH, and BiVO₄/NiOOH weremeasured using a zeta potential analyzer (Micromeritics NanoPlus-2)after dispersing nanoparticles scratched from each electrode in 0.5 Mphosphate buffer solution (pH 7). To compensate for the loss of the OEClayer and the reduction of the OEC coverage on the BiVO₄ surface duringsample preparation (e.g., sonication) a thicker layer of OEC wasdeposited for this measurement.

That the photoelectrochemical performances of the electrodes for sulfiteoxidation and water oxidation are summarized in Tables 2 and 3, whichare discussed in more detail in the Results.

TABLE 1 Specific surface area Slit Pore (m² g⁻¹) V_(total) V_(micro)V_(meso) Width size Sample S_(BET) S_(Langmuir) S_(micro) (mL g⁻¹) (mLg⁻¹) (mL g⁻¹) (Å) (nm) Nanoporous 31.8 ± 2.3 42.5 ± 2.7 13.3 0.051 0.0110.041 8.9 20~250 BiVO₄ (0.9978)* (0.9998) *The values in parenthesesdenote correlation factors. Parameters of the nanoporous BiVO₄ obtainedfrom N₂ adsorption-desorption measurement; S_(BET) = specific surfacearea calculated from BET equation, S_(Langmuir) = specific surface areacalculated from Langmuir equation, V_(total) = total pore volume (takenfrom the volume of N₂ adsorbed at about P/P_(o) = 0.990), V_(micro) =micropore volume (estimated by the t-plot), slit width estimated by themicropore analysis (MP) method, and the average pore diameter estimatedfrom the Barrett Joyner-Halenda (BJH) formula.

Photoelectrochemical Measurements

Photoelectrochemical performances of photoanodes were evaluated in atypical undivided three-electrode configuration using a SP-200potentiostat/EIS (BioLogic Science Instrument). The simulated solarillumination was obtained by passing light from a 300 W Xe arc lampthrough a water filter (IR filter)/neutral density filters/an AM 1.5 Gfilter. Illumination through the FTO side (back-side illumination) wasused. The power density of the incident light was calibrated to 100mW/cm² at the surface of the FTO substrate (before the light penetratesFTO) by using a thermopile detector (International Light) and a NRELcertified reference cell (Photo Emission Tech., Inc.). All illuminatedareas were 0.2 cm². Photocurrent measurements were performed in a 0.5 Mpotassium phosphate (KH₂PO₄) buffer solution (pH 7) with or without 1 Msodium sulfite (Na₂SO₃) as a hole scavenger. Prior to measurements, theelectrolyte was thoroughly deaerated by purging it with nitrogen for 1h. Photocurrents were monitored either while sweeping the potential tothe positive direction with a scan rate of 10 mV/s or while applying aconstant bias. Photocurrents were also obtained with choppedillumination to examine 7 transient photocurrents. While allmeasurements were carried out using a Ag/AgCl (4M KCl) referenceelectrode, all results in this work were presented against thereversible hydrogen electrode (RHE) for ease of comparison with H₂ andO₂ redox levels and other reports that used electrolytes with differentpH conditions. The conversion between potentials vs. Ag/AgCl and vs. RHEis performed using the equation below.E (vs. RHE)=E (vs. Ag/AgCl)+E_(Ag/AgCl) (reference)+0.0591 V×pH(E_(Ag/AgCl) (reference)=0.1976 V vs. NHE at 25° C.).

Incident photon-to-current efficiency (IPCE) at each wavelength wasdetermined using illumination from a 300 W Xe arc lamp passed through anAM 1.5 G filter and neutral density filters to approximate the output ofthe sun. Monochromatic light was produced using an Oriel Cornerstone 130monochromator with a 10-nm bandpass, and the output was measured with aphotodiode detector. IPCE was measured at 0.6 V vs. RHE in 0.5 Mphosphate buffer (pH 7) using the same three-electrode setup describedabove for photocurrent measurements. Absorbed photon-to-currentefficiency (APCE) was obtained by dividing the IPCE by the lightharvesting efficiency (LHE) at each wavelength using the followingequations.APCE (%)=IPCE (%)/LHELHE=1-10^(−A(λ)) (A(λ): absorbance at wavelength λ)

In order to calculate J_(max) (maximum photocurrent density achievableassuming 100% IPCE for photons with energy ≧E_(g)) of BiVO₄ with abandgap energy of 2.5 eV, the National Renewable Energy Laboratory(NREL) reference solar spectral irradiance at AM 1.5 G (radiation energy(W·m⁻²·nm⁻¹) vs. wavelength (nm)) (ASTM Standard G173-03, 2008,“Standard Tables for Reference Solar Spectral Irradiances: Direct Normaland Hemispherical on 37° Tilted Surface,” ASTM International, WestConshohocken, Pa., 2003, DOI: 10.1520/G0173-03R08) was first convertedto the solar energy spectrum in terms of number of photons(s⁻¹·m⁻²·nm⁻¹) vs. wavelength (nm). Then, the number of photons abovethe bandgap energy of the bare-BiVO₄ shown in this study (bandgap=2.5eV) was calculated using a trapezoidal integration (in 10 nm increments)of the spectrum and was converted to the current density (mA·cm⁻²). Inorder to calculate J_(abs) (photocurrent assuming 100% APCE), the LHE ateach wavelength was multiplied during each step of the trapezoidalintegration. Using these calculations, J_(max)=6.47 mA/cm² andJ_(abs)=4.45 mA/cm² were obtained.

The STH efficiency was calculated using a J-V curve obtained from a twoelectrode system assuming 100% Faradaic efficiency using the followingequation, where J is the photocurrent density, V_(bias) is the appliedbias between WE and CE, and P_(in) is the incident illumination powerdensity (AM 1.5G, 100 mW/cm²). (See M. G. Walter, E. L. Warren, J. R.McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Solar WaterSplitting Cells. Chem. Rev. 110, 6446-6473 (2010).)

${{STH}\mspace{14mu}{Efficiency}\mspace{14mu}(\%)} = {\left\lbrack \frac{{J\left( \frac{mA}{{cm}^{2}} \right)} \times \left( {1.23\text{-}V_{bias}} \right)(V)}{P_{in}\left( \frac{mW}{{cm}^{2}} \right)} \right\rbrack_{{AM}\mspace{11mu} 1.5G} \times 100}$

Capacitances were measured to obtain Mott-Schottky plots for the BiVO₄based electrodes using a SP-200 potentiostat/EIS (BioLogic ScienceInstrument). A sinusoidal modulation of 10 mV was applied at frequenciesof 0.5 and 1 kHz. The three-electrode setup was used with a 0.5 Mphosphate buffer (pH 7). All electrodes were masked with epoxy resin toexpose the same geometrical area (0.2 cm²) for this measurement.

O₂ measurements were conducted using an Ocean Optics fluorescence-basedoxygen sensor (FOSPOR-R 1/16″). The probe measures the O₂ content in theheadspace as mole %, which was converted to micromoles after adjustingfor the O₂ dissolved in solution using Henry's Law. Two different cellconfigurations were tested for O₂ measurements. One was a custom built,airtight, two-compartment cell divided by a frit with one chambercontaining a BiVO₄/FeOOH/NiOOH WE along with a Ag/AgCl RE and the othercontaining a Pt coil CE (three-electrode setup). For this cell, O₂measurements were made while applying 0.6 V between the WE and the CE.The amount of O₂ detected was divided by the amount of O₂ expectedcalculated from photocurrent assuming 100% Faradaic efficiency (F.E.) tocalculate the true Faradaic efficiency or the photocurrent-to-O₂conversion efficiency. Measurements obtained from the two differentconfigurations gave comparable results (F.E.>90%). H₂ measurements werecarried out by applying 0.6 V between the WE and the CE (two-electrodesystem) in an airtight undivided cell, using gas chromatography (GC)(SRI Instruments) to analyze the headspace. The amount of H₂ gas evolvedwas determined by taking 100 μL of gas from the headspace of the cellusing a syringe and injecting it into the gas-sampling loop of the GCevery three hours. The GC was equipped with a packed MolSieve 13Xcolumn. Helium (Airgas, ultra high purity) was used as the carrier gas.A helium ionization detector (HID) was used to quantify hydrogenconcentration.

Additional information regarding the materials, methods andcharacterization of the electrodes can be found in Choi et al.,Nanoporous BiVO₄ Photoanode with Dual-Layer Oxygen Evolution Catalystfor Solar Water Splitting, Science 343, 990-994 (2014), including theaccompanying Supplementary Material available atwww.sciencemag.org/content/science.1246913/DCI, the entire contents ofwhich are incorporated herein by reference.

Results

The use of comparatively hydrophobic VO(acac)₂/DMSO solution overcamethis problem and resulted in a remarkable increase in surface area. Thetop view and side-view scanning electron microscopy (SEM) images showthe formation of much smaller BiVO₄ nanoparticles (mean particlesize=76±5 nm) (FIG. 5) creating a 3D nanoporous network (FIG. 2, D toF).

N₂ adsorption-desorption-isotherm measurements show that the nanoporousBiVO₄ electrode contains micropores within BiVO₄ particles as well asmesopores and macropores between BiVO₄ nanoparticles (FIG. 6 and Table1). The specific surface area of the nanoporous BiVO₄ electrode wasestimated to be 31.8±2.3 m²/g based on a fitting analysis using theBrunauer-Emmett-Teller equation. (T. Allen, Particle Size Measurement(Chapman and Hall, London, 1997).) BiVO₄ electrodes prepared by usingother synthesis methods (such as metal organic decomposition, spraydeposition, or direct electrodeposition of BiVO₄) possess more limitedsurface areas.

The purity and crystal structure of the nanoporous BiVO₄ electrode(monoclinic scheelite structure) were confirmed with x-ray diffraction.The bandgap of the nanoporous BiVO₄ electrode was estimated to be ˜2.50to 2.55 eV, using ultraviolet-visible absorption spectra.

The photoelectrochemical properties were first examined in the presenceof 1 M sodium sulfite (Na₂SO₃), which served as the hole scavenger. Theoxidation of sulfite is thermodynamically and kinetically more facilethan oxidation of water, and therefore, measuring photocurrent forsulfite oxidation enables investigation of the photoelectrochemicalproperties of BiVO₄ independently of its poor water oxidation kinetics.A typical photocurrent-potential (J-V) curve of the sulfite oxidationwith nanoporous BiVO₄ is shown in FIG. 7A. A very early photocurrentonset potential, 0.1 V versus reversible hydrogen electrode (RHE), and arapid increase in photocurrent in the 0.2 V<E versus RHE<0.6 V region,representing an excellent fill factor, resulted in a photocurrentdensity of 3.3±0.3 mA/cm² at a potential as low as 0.6 V versus RHE. Theincident photon-to-current conversion efficiency (IPCE) and the absorbedphoton-to-current conversion efficiency (APCE) of the nanoporous BiVO₄at 0.6 V versus RHE are 60 and 72%, respectively, at 420 nm (FIG. 7B).

Photocurrent density obtained for sulfite oxidation was used tocalculate φ_(sep) by using Eq. 1, where J_(PEC) is the measuredphotocurrent density and J_(abs) is the photon absorption rate expressedas current density, which is calculated assuming 100% APCE, as describedpreviously. J_(abs) of the nanoporous BiVO₄ electrode was calculated tobe 4.45 mA/cm². φ_(sep) is the yield of the photogenerated holes thatreach the surface, and φ_(ox) is the yield of the surface reaching holesthat are injected into the solution species. (M. Zhou et al., Chem SusChem 5, 1420-1425 (2012).)J _(PEC) =J _(abs)×φ_(sep)×φ_(ox)  Eq. (1)For sulfite oxidation with extremely fast oxidation kinetics, surfacerecombination is negligible, and φ_(ox) is ˜1. Therefore, φ_(sep) isobtained by dividing J_(PEC) by J_(abs) (FIG. 7A, inset). The resultshows that the nanoporous BiVO₄ electrode achieves φ_(sep)=0.70±0.03 and0.90±0.03 at 0.6 V and 1.23 V versus RHE, respectively. The holediffusion length of BiVO₄ was recently reported to be ˜100 nm when usingsingle-crystal BiVO₄. (A. J. E. Rettie et al., J. Am. Chem. Soc. 135,11389-11396 (2013).) The mean particle size of BiVO₄ composing thenanoporous BiVO₄ electrode shown in FIG. 2D is 76±5 nm (FIG. 5), and theparticle size obtained from the XRD peaks when using the Scherrerequation is 27±2 nm. Therefore, the nanoporosity incorporated into BiVO₄electrodes in this study appears to be ideal for effectively suppressingbulk carrier recombination, resulting in a record high φ_(sep).

Photocurrent from the nanoporous BiVO₄ for water oxidation shown in FIG.3A (black line) is considerably lower than the photocurrent for sulfiteoxidation (FIG. 7A), indicating that the majority of thesurface-reaching holes were lost to surface recombination because of thepoor catalytic nature of the BiVO₄ surface for water oxidation. (Y.Park, K. J. McDonald, K.-S. Choi, Chem. Soc. Rev. 42, 2321-2337 (2013).)To improve water oxidation kinetics, a thin FeOOH or NiOOH layer wasphotodeposited on the nanoporous BiVO₄ surface as an OEC layer in orderto assemble BiVO₄/FeOOH and BiVO₄/NiOOH electrodes. Their thicknesseswere optimized so as to maximize photocurrent generation. It has beenpreviously demonstrated that FeOOH interfaces well with BiVO₄ (K. J.McDonald, K.-S. Choi, Energy Environ. Sci 5, 8553-8557 (2012) and J. A.Seabold, K.-S. Choi, J. Am. Chem. Soc. 134, 2186-2192 (2012)), whereasNiOOH is known to be a more active OEC than FeOOH (less overpotentialrequired to achieve the same current density) as a dark electrocatalyston a conducting substrate. (L. Trotochaud, J. K. Ranney, K. N. Williams,S. W. Boettcher, J. Am. Chem. Soc. 134, 17253-17261 (2012); R. D. L.Smith et al., Science 340, 60-63 (2013); and M. W. Louie, A. T. Bell, J.Am. Chem. Soc. 135, 12329-12337 (2013).)

The photocurrents for water oxidation from the resulting BiVO₄/FeOOH andBiVO₄/NiOOH photoanodes were markedly higher than those from the bareBiVO₄ electrode (FIG. 3A and Table 3), but their photocurrents werestill lower than that generated for sulfite oxidation at the bare BiVO₄electrode. This comparison suggested that neither BiVO₄/FeOOH norBiVO₄/NiOOH engages all surface-reaching holes in the oxygen evolutionreaction, instead losing a portion to surface recombination at theBiVO₄/OEC junction. The interface states formed at the BiVO₄/OECjunction can serve as recombination centers and cause surfacerecombination. The fact that BiVO₄/FeOOH generated higher photocurrentthan did BiVO₄/NiOOH, although NiOOH shows faster water oxidationkinetics as an electrocatalyst, suggests that the interfacerecombination at the BiVO₄/NiOOH junction is more substantial than thatat the BiVO₄/FeOOH junction. This can be easily confirmed by comparingphotocurrents for sulfite oxidation by BiVO₄, BiVO₄/FeOOH, andBiVO₄/NiOOH (FIG. 3, B and C, and Table 3). Because the interfacial holetransfer rates for sulfite oxidation on the BiVO₄, FeOOH, and NiOOHsurfaces should be equally fast, any difference observed inphotocurrents for sulfite oxidation by BiVO₄, BiVO₄/FeOOH, andBiVO₄/NiOOH should be mainly due to the recombination at the BiVO4/OECjunction. The comparison shows that photocurrent for BiVO₄/FeOOH is veryclose to that for BiVO₄, whereas the photocurrent for BiVO₄/NiOOH isconsiderably lower, which indicates that the interface recombination atthe BiVO₄/NiOOH junction is indeed more substantial.

In addition to the interface recombination at the BiVO₄/OEC junction,slow water oxidation kinetics at the OEC/solution junction can causeadditional surface recombination during water oxidation. (C. Y.Cummings, F. Marken, L. M. Peter, A. A. Tahir, K. G. Wijayantha, Chem.Commun. (Camb.) 48, 2027-2029 (2012); and L. M. Peter, K. G. Wijayantha,A. A. Tahir, Faraday Discuss. 155, 309-322, discussion 349-356 (2012).)This additional surface recombination can be shown as the difference inphotocurrent for sulfite oxidation and water oxidation (FIG. 3, B andC). When the rate of interfacial hole transfer for water oxidation isslower than the rate of holes entering the OEC layer, holes areaccumulated in the OEC layer and at the BiVO₄/OEC junction, which inturn increases the electron current from the CB of BiVO₄ to the OEClayer for surface recombination. (L. M. Peter, K. G. Wijayantha, A. A.Tahir, Faraday Discuss. 155, 309-322, discussion 349-356 (2012).)Because FeOOH has slower water oxidation kinetics than that of NiOOH,the difference in photocurrent for water oxidation and sulfite oxidationis more pronounced for BiVO₄/FeOOH than BiVO₄/NiOOH (FIG. 3, B and C).However, when the effects of interface recombination at the BiVO₄/OECjunction and water oxidation kinetics are combined, BiVO₄/NiOOH losesmore surface-reaching holes to surface recombination and generates lowerphotocurrent than does BiVO₄/FeOOH for water oxidation (FIG. 3A).

On the basis of this new understanding of the BiVO₄/OEC and theOEC/electrolyte interfaces, consecutive layers of FeOOH and NiOOH weredeposited, simultaneously optimizing the BiVO₄/OEC and theOEC/electrolyte junctions. The FeOOH at the BiVO₄/OEC junction willreduce the interface recombination, whereas the NiOOH at theOEC/electrolyte junction will realize faster water oxidation kineticsthan if FeOOH was used as the outermost layer.

The BiVO₄/FeOOH/NiOOH and BiVO₄/FeOOH show comparable J-V curves forsulfite oxidation, confirming that the BiVO₄/FeOOH junction effectivelyreduces the interface recombination at the BiVO₄/OEC junction (FIG. 3, Band D). As a result, BiVO₄/FeOOH/NiOOH shows impressive overallperformance for water oxidation, reaching a photocurrent density of2.8±0.2 mA/cm² at 0.6 V versus RHE (FIG. 3A and Table 2), which ismarkedly better than those of BiVO₄/FeOOH and BiVO₄/NiOOH and is almostcomparable with the performance of bare BiVO₄ for sulfite oxidation.

When NiOOH was first deposited on the BiVO₄ surface and FeOOH was addedas the outermost layer to form BiVO₄/NiOOH/FeOOH (reversed OECjunction), the J-V curve for sulfite oxidation by BiVO₄/NiOOH/FeOOH wascomparable with that by BiVO₄/NiOOH, confirming that a BiVO₄/NiOOHjunction is not favorable for interface recombination (FIG. 3, C and E).As a result, BiVO₄/NiOOH/FeOOH showed the lowest photocurrent for wateroxidation. These results demonstrate that the photocurrent enhancementachieved by the BiVO₄/FeOOH/NiOOH photoanode for photoelectrolysis ofwater is truly due to the simultaneous optimization of the BiVO₄/OEC andOEC/electrolyte junctions, using an optimum dual OEC structure.

TABLE 2 Average photocurrent onset potential and photocurrent densitiesat 0.6 and 1.23 V vs. RHE obtained from J-V measurements for wateroxidation for BiVO₄, BiVO₄/FeOOH, BiVO₄/NiOOH, BiVO₄/ FeOOH/NiOOH, andBiVO₄/NiOOH/FeOOH in a 0.5M phosphate buffer (pH 7) solution (AM 1.5G,100 mW/cm²). For each system five measurements using five differentsamples were used for analysis. BiVO₄/ BiVO₄/ BiVO₄/ BiVO₄/ FeOOH/NiOOH/ BiVO₄ FeOOH NiOOH NiOOH FeOOH Photocurrent 0.43 0.31 0.26 0.230.31 onset potential (±0.04) (±0.03) (±0.04) (±0.03) (±0.02) (V vs. RHE)J at 0.6 V 0.4 2.2 1.8 2.8 1.6 vs. RHE (±0.2) (±0.2) (±0.2) (±0.2)(±0.2) (mA/cm²) J at 1.23 V 1.8 3.6 3.3 4.2 3.2 vs. RHE (±0.3) (±0.2)(±0.3) (±0.3) (±0.4) (mA/cm²)

TABLE 3 Average photocurrent onset potential and photocurrent density at0.6 V vs. RHE obtained from J-V measurements for sulfite oxidation forBiVO₄, BiVO₄/FeOOH, BiVO₄/NiOOH, BiVO₄/ FeOOH/NiOOH, andBiVO₄/NiOOH/FeOOH in a 0.5M phosphate buffer (pH 7) solution containing1M Na₂SO₃ (AM 1.5G, 100 mW/cm²). For each system five measurements usingfive different samples were used for analysis. BiVO₄/ BiVO₄/ BiVO₄/BiVO₄/ FeOOH/ NiOOH/ BiVO₄ FeOOH NiOOH NiOOH FeOOH Photocurrent 0.110.18 0.12 0.13 0.19 onset potential (±0.02) (±0.02) (±0.02) (±0.02)(±0.02) (V vs. RHE) J at 0.6 V 3.3 3.1 2.4 3.1 2.4 vs. RHE (±0.3) (±0.2)(±0.2) (±0.2) (±0.3) (mA/cm²)

The STH efficiency of the BiVO₄/FeOOH/NiOOH electrode calculated byusing its J-V curve obtained by using a two-electrode system (workingelectrode and a Pt counter electrode), assuming 100% Faradaicefficiency, is plotted in FIG. 4A. (M. G. Walter et al., Chem. Rev. 110,6446-6473 (2010).)) The maximum STH efficiency of 1.72% achieved by thesystem is impressive because it is obtained by using unmodified BiVO₄ asa single photon absorber. Moreover, this efficiency is achieved at apotential as low as 0.58 V versus RHE, which is a highly favorablefeature for assembling a tandem cell or a photoelectrochemical diode.(F. F. Abdi et al., Nat. Commun. 4, 2195 (2013), M. G. Walter et al.,Chem. Rev. 110, 6446-6473 (2010) and L. Tong et al., Energy Environ.Sci. 5, 9472-9475 (2012).) The long term stability of BiVO₄/FeOOH/NiOOHwas tested by obtaining a J-t curve. A photocurrent density of 2.73mA/cm², obtained by applying 0.6 V between the working and counterelectrodes, was maintained for 48 hours without showing any sign ofdecay, proving its long-term stability (FIG. 4B). The O₂ measurementmade by using a fluorescence O₂ sensor confirmed that the photocurrentgenerated at 0.6 V versus counter electrode was mainly associated withO₂ production (>90% photocurrent-to-O₂ conversion efficiency) (FIG. 4C).The same results were obtained when the measurement was performed at 0.6V versus RHE. H₂ production at the Pt counter electrode was alsodetected with gas chromatography (GC) (FIG. 4C). The molar ratio of theproduced H₂:O₂ was 1.85:1. The slight deviation from the stoichiometricratio of 2:1 is due to our imperfect manual sampling method of H₂ for GCanalysis.

Because this outstanding performance was achieved by using simple,unmodified BiVO₄ (no extrinsic doping and no composition tuning) as theonly photon absorber, further improvement of the cell efficiency can beachieved by tuning compositions or forming heterojunctions and tandemcells are used to enhance photon absorption and electron-holeseparation.

Unless otherwise specified, all temperature- and/or pressure-dependentvalues cited herein are the values at the temperature and pressure atwhich the electrodes or PECs are being operated. For example, the anelectrode or PEC may be operated at room temperature (˜23° C.) andatmospheric pressure.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. An electrode comprising a porous network ofconnected BiVO₄ particles, including BiVO₄ particles that are merged attheir interfaces, wherein the porous network has a surface area of atleast about 20 m²/g, as measured by the Brunauer-Emmett-Teller method,and further wherein the electrode is characterized in that it generatesa photocurrent density of at least 0.2 mA/cm² at 1 V versus reversiblehydrogen electrode for water oxidation under 100 mW/cm², AM 1.5 Gillumination.
 2. The electrode of claim 1 having an electron-holeseparation yield of at least 0.8 at 1.23 V versus a reversible hydrogenelectrode.
 3. The electrode of claim 1, further comprising a coating ofoxygen evolution catalyst on the surfaces of the BiVO₄ particles.
 4. Theelectrode of claim 3, wherein the oxygen evolution catalyst comprisesNiO_(x)(OH)_(y) (0≦x≦1.5, 0≦y≦3, where at least one of x and y has avalue >0) or FeO_(x)(OH)_(y) (0≦x≦1.5, 0≦y≦3, where at least one of xand y has a value >0), or a combination thereof.
 5. The electrode ofclaim 1, wherein the BiVO₄ comprises vacancies at a portion of theoxygen atom sites and further wherein a portion of the oxygen atoms inthe BiVO₄ are substituted with nitrogen atoms.
 6. The electrode of claim5, wherein the nitrogen atoms reduce the bandgap of the BiVO₄ relativeto the BiVO₄ without the nitrogen atoms.
 7. The electrode of claim 1,wherein the porous network has a surface area of at least about 25 m²/g,as measured by the Brunauer-Emmett-Teller method.
 8. The electrode ofclaim 1, wherein the BiVO₄ particles have a mean particle size of lessthan about 100 nm.
 9. The electrode of claim 1, wherein the BiVO₄particles have a mean particle size of less than about 150 nm.
 10. Anelectrode comprising: a porous network of connected BiVO₄ particles,including BiVO₄ particles that are merged at their interfaces, whereinthe porous network has a surface area of at least about 10 m²/g, asmeasured by the Brunauer-Emmett-Teller method; and a coating of oxygenevolution catalyst on the surfaces of the BiVO⁴ particles, wherein thecoating of oxygen evolution catalyst comprises an inner layer of a firstoxygen evolution catalyst in direct contact with the surfaces of theBiVO₄ particles and an outer layer of a second oxygen evolution catalystdisposed on the inner layer, wherein the first oxygen evolution catalystis characterized in that it creates fewer interfacial states that canserve as recombination centers at the BiVO₄/oxygen evolution catalystinterface than would the second oxygen evolution catalyst and the secondoxygen evolution catalyst is characterized in that it has highercatalytic activity for oxygen evolution than does the first oxygenevolution catalyst.
 11. The electrode of claim 10, wherein the firstoxygen evolution catalyst is FeO_(x)(OH)_(y) (0≦x≦1.5, 0≦y≦3, where atleast one of x and y has a value >0) and the second oxygen evolutioncatalyst is NiO_(x)(OH)_(y) (0≦x≦1.5, 0≦y≦3, where at least one of x andy has a value >0).
 12. An electrode comprising BiVO₄ particles and acoating of oxygen evolution catalyst on the surfaces of the BiVO₄particles, wherein the coating of oxygen evolution catalyst comprises aninner layer of a first oxygen evolution catalyst in direct contact withthe surfaces of the BiVO₄ particles and an outer layer of a secondoxygen evolution catalyst disposed on the inner layer, wherein the firstoxygen evolution catalyst is characterized in that it creates fewerinterfacial states that can serve as recombination centers at theBiVO₄/oxygen evolution catalyst interface than would the second oxygenevolution catalyst and the second oxygen evolution catalyst ischaracterized in that it has higher catalytic activity for oxygenevolution than does the first oxygen evolution catalyst.
 13. Thematerial of claim 12, wherein the first oxygen evolution catalyst isFeO_(x)(OH)_(y) (0≦x≦1.5, 0≦y≦3, where at least one of x and y has avalue >0) and the second oxygen evolution catalyst is NiO_(x)(OH)_(y)(0≦x≦1.5, 0≦y≦3, where at least one of x and y has a value >0).
 14. Aphotoelectrochemical cell comprising: a working electrode comprising aporous network of connected BiVO₄ particles, including BiVO₄ particlesthat are merged at their interfaces, wherein the porous network has asurface area of at least about 20 m²/g, as measured by theBrunauer-Emmett-Teller method, and further wherein the working electrodeis characterized in that it generates a photocurrent density of at least0.2 mA/cm² at 1 V versus reversible hydrogen electrode for wateroxidation under 100 mW/cm², AM 1.5 G illumination; a counter electrodein electrical communication with the working electrode; and anelectrolyte solution in which the working electrode and the counterelectrode are immersed.
 15. A method for electrochemical reactions usinga photoelectrochemical cell comprising: a working electrode comprising aporous network of connected BiVO₄ particles, including BiVO₄ particlesthat are merged at their interfaces, wherein the porous network has asurface area of at least about 20 m²/g, as measured by theBrunauer-Emmett-Teller method, and further wherein the working electrodeis characterized in that it generates a photocurrent density of at least0.2 mA/cm² at 1 V versus reversible hydrogen electrode for wateroxidation under 100 mW/cm², AM 1.5 G illumination; a counter electrodein electrical communication with the working electrode; and anelectrolyte solution in which the working electrode and the counterelectrode are immersed, the method comprising: exposing the BiVO₄electrode to radiation having energy greater than the bandgap of theBiVO₄, such that: the BiVO₄ particles absorb the radiation to produceelectron-hole pairs; the holes are transported to the electrolyte-BiVO₄interface where they undergo oxidation reactions with the electrolyte;and the electrons are transported to the counter electrode where theyundergo reduction reactions with the electrolyte.
 16. The method ofclaim 15, wherein the electrolyte solution is an aqueous electrolytesolution; the holes at the electrolyte-BiVO₄interface oxidize water inthe aqueous electrolyte solution to form O₂; and the electrons at thecounter electrode reduce water in the aqueous electrolyte solution toform H₂.
 17. A method of making an electrode, the method comprising:applying an organic solution comprising vanadium-containing precursormolecules to the surface of an electrode comprised of plate-like BiOXcrystals, where X represents a halogen atom or a mixture of halogenatoms; and heating the organic solution-covered electrode comprised ofplate-like BiOX crystals to form an electrode comprising a porousnetwork of connected BiVO₄ particles, including BiVO₄particles that aremerged at their interfaces; wherein the porous network of connectedBiVO₄ particles has a surface area of at least about 20 m²/g, asmeasured by the Brunauer-Emmett-Teller method, and further wherein theelectrode is characterized in that it generates a photocurrent densityof at least 0.2 mA/cm² at 1 V versus reversible hydrogen electrode forwater oxidation under 100 mW/cm², AM 1.5 G illumination.
 18. The methodof claim 17, where X represents iodine.
 19. The method of claim 17,further comprising applying a coating of an oxygen evolution catalyst tothe surface of the porous network of connected BiVO₄ particles.
 20. Themethod of claim 17, further comprising exposing the porous network ofconnected BiVO₄ particles to an elevated temperature in a reducing orinert atmosphere to create oxygen vacancies in the BiVO₄.